Few studies have examined the effects of extensive reading (ER) on foreign language learners' acquisition of sight vocabulary, especially for partially known high-frequency words and reading fluency development. This study compared groups of non-English-major Japanese university-level students engaged in ER with intensive reading (IR) for two semesters. There were two ER Groups--one that read over 50,000 words (n = 21) and one that read below 50,000 words (n = 26)--and one IR Group (n = 25). The participants spent almost the same time on their task and were exposed mainly to 1k- and 2k-level vocabulary. The results of the vocabulary level tests revealed that ER was more effective and efficient for developing sight vocabulary of partially known high-frequency words. Only the ER Groups significantly improved their reading rates, indicating that ER facilitated reading fluency better than IR; moreover, the more words they read, the greater their improvement. The pedagogical implication is that the adoption of ER in an EFL curriculum is a feasible option to facilitate vocabulary acquisition and reading fluency for communication purposes in the classroom.